Modems are used for transmitting data over communication links. Generally, two modems on opposite ends of a communication link send each other data by converting the data into electrical signals suitable for transmission on the link. In order to allow modems of different vendors to transmit data to each other, standards have been defined stating exactly how the signals should be modulated. These standards include, for example, the V.34 standard which allows transmission on both analog and digital links, the V.90 standard which is defined for connections between clients connected to analog lines, such as home users, and servers connected to digital lines (e.g., T1/E1 lines), such as Internet service providers, and the V.91 standard which is meant for pure digital connections.
The V.34 standard is described in “A modem operating at data signalling rates of up to 33 600 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits”, ITU-T Recommendation V.34, 2/98, the disclosure of which is incorporated herein by reference.
The V.90 standard is described in “A digital modem and analogue modem pair for use on the public switched telephone network (PSTN) at data signalling rates of up to 56 000 bit/s downstream and up to 33 600 bit/s upstream”, ITU-T Recommendation V.90, 9/98, the disclosure of which is incorporated herein by reference.
The V.91 standard is described in “A digital modem operating at data signalling rates of up to 64 000 bit/s for use on a 4-wire circuit switched connection and on leased point-to-point 4-wire digital circuits”, ITU-T Recommendation V.91, 5/99, the disclosure of which is incorporated herein by reference.
The V.34 standard allows transmission rates of up to 33.6 Kbps in both directions. The V.90 standard allows transmission at a rate of up to 56 Kbps in one direction (referred to as the downstream direction) and 33.6 Kbps in the other direction. The V.91 standard allows transmission at 64 Kbps in both directions. The V.91 standard, however, is relatively new and it is expected to take many years, if at all, until most of the modems in the market implement the V.91 standard.
Usually modems implement a plurality of different standards. Generally, when two modems form a connection they search for a highest transmission rate standard which they both implement and use this standard for transmission over the connection. The search for a common transmission standard is usually performed in accordance with the V.8 standard.
FIG. 1 is a schematic illustration of a V.90 connection as is known in the art. A client modem 12, connected to a public switching telephone network (PSTN) 19 through analog lines 14, forms a connection with a server modem 24 which is connected to PSTN 19 through a digital link 32. Usually, modem 24 belongs to a modem pool, for example of an Internet service provider (ISP) 29. Generally, a line card 16 translates the signals from analog lines 14 to a digital link 36 of PSTN 19, and vice versa. It is noted, that in most cases, except for analog lines 14 which connect line card 16 and client modem 12, PSTN 19 is formed of substantially only digital links, represented in FIG. 1 by a digital network 23.
Client modem 12 comprises a signal processing unit 13 and a sampler and reconstructer 15 which turns digital signals into analog signals for transmission and analog signals from lines 14 into digital signals. When a computer 10 requests to a connection to ISP 29, client modem 12 forms a negotiation V.8 connection with server modem 24. During the negotiation connection, client modem 12 identifies as an analog modem and server modem 24 identifies as a digital modem such that the modems agree to use a V.90 connection for transmission of data. Thereafter, modems 12 and 24, sequentially as defined by the V.90 protocol, transmit test signals used to check the characteristics of links 14 and 32.
After the tests are concluded, modems 12 and 24 move into transceiving states according to the V.90 standard. Signals transmitted from computer 10, are prepared for transmission by processing unit 13 of modem 12 at a rate of up to 33.6 Kbps and are then converted to analog signals by a D/A of sampler and reconstructer 15. Signals transmitted by server modem 24 are transmitted from the modem at a rate of up to 56 Kbps as described above. The transmitted signals from server modem 24 are converted to analog signals by line card 16 and are passed on link 14 to modem 12. An A/D of sampler and reconstructer 15 of modem 12 samples the analog signals at a high enough rate which allows proper operation of the modem, i.e., proper synchronization of a clock of sampler and reconstructer 15 in receiving modem 12 to the timing of transmitting modem 24. Generally, to allow for rate correction, the A/D of sampler and reconstructer 15 samples the signals at a rate higher than 8000 samples per second. The sampled digital signals are then passed to processing unit 13 for processing.
FIG. 2 is a schematic block diagram of an exemplary receiving path 40 of processing unit 13 of modem 12, as is known in the art. Path 40 receives samples from sampler and reconstructer 15 on a line 42. The samples are added to correction values provided by an echo canceller 44 and are then filtered by a channel filter 46. The filtered samples are passed to a timing recovery unit 48 and a rate converter 50, which correct for timing drifts of the received samples. The samples are passed through an automatic gain control (AGC) unit 52 and are then provided to an equalizer 54 which corrects phase and amplitude distortions of the received samples. The samples from equalizer 54 are passed to a symbol decision module 56 which determines for each sample which symbol it represents. Symbol decision module 56 also detects attenuation pad impairments and performs robbed bit signaling (RBS) in order to better perform the determination of the samples which represent the symbols. The symbols are then passed through a symbol to bit converter 58 which translates the symbols into bits, and through a descrambler 60 which descrambles the bits.
The definition of the V.90 connection is such that it can only be established between an analog connected client (as most client modems are home modems connected through analog wires to the public switching telephone network (PSTN)) and a digitally connected server modem (as most server modems belong to ISPs which are connected digitally to the PSTN). In recent years, many computers connect to ISPs through all-digital connections, for example, using cellular phone connections which are entirely digital. Until the V.91 protocol is widely implemented, these computers must use, at most, the V.34 protocol which provides a maximal transmission rate of 33.6 Kbps in both the upstream and downstream.